Temporal responses of community fine root populations to long-term elevated atmospheric CO2 and soil nutrient patches in model tropical ecosystems

Biomass and length density of fine roots, as well as overall allocation of dry matter to root growth, of C-3 plants has been shown to increase under elevated CO2. However, it is uncertain whether the stimulatory effect of elevated CO2 on fine root population size in plant communities will persist, or whether fine root populations at high CO2 simply reach their maximum sooner (or possibly later) than those produced under ambient CO2. It is also unclear whether increased nutrient demand at the stand-level under elevated CO2 will lead to more intense nutrient foraging via enhanced fine root proliferation into relatively nutrient-rich soil microsites. I addressed these questions in a 530 day experiment with model tropical plant communities established in four equivalent ecosystem (17 m(3)) in which plants shared a common low fertility soil. Fine root (less than or equal to 2 mm empty set) populations (biomass and length density) in ecosystems maintained at elevated CO2 (610 mu l l(-1)) increased more rapidly than those in ecosystems maintained at ambient CO2 (340 mu l l(-1)) during the first half of the experiment and also remained greater over the entire experiment. The data also indicate that: (1) fine root populations at both CO2 levels eventually stabilize, (2) stabilization occurs sooner under elevated CO2 (occupation of the soil volume), and (3) steady-state populations under elevated CO2 may be slightly larger than those maintained under ambient CO2. Fine root proliferation into artifically nutrient-enriched microsites was dramatic in all ecosystems (22% to 75% greater than into non-enriched soil). However, proliferation into enriched microsites was not enhanced by elevated CO2. Thus, elevated CO2 may not enhance exploitation of nutrient-rich microsites even in low fertility soils, suggesting that increased plant nutrient capture under elevated CO2 also may be unlikely.